Identification of intracellular and cell surface molecules responsible for susceptibility to neurovirulent wild type and vaccine viruses will provide information to help genetically engineer non-neurovirulent vaccines. Using a human cell line that differentiates from fibroblast to neuron (NT2 ) we will examine neuron-specific components linked to neurovirulence. Before and after differentiation, NT2 cells will be infected with parental or neurovirulent variants of non-neurovirulent (e.g., Jeryl Lynn vaccine), or parental or non-neurovirulent variants of neurovirulent (e.g., Kilham wild type) vaccine and wild type mumps virus. Differences in replication (e.g., virus binding, titer of infectious virions, and viral proteins and RNA) will be compared between parental wild type and vaccine viruses and their variants before (fibroblastic) and after (neuronal) differentiation. Cell surface analysis, cell fractionation and gel shift analysis will identify specific components interacting with neurovirulent viruses after neuronal differentiation of cells. Brain damage due to intrauterine virus infections is the most common congenital defect in children. In addition, the infant's brain continues to develop during the first year of postnatal life. Since the developing nervous system is uniquely sensitive to damage following virus infection, administering neurovirulent vaccines to infants also places the child's CNS at increased risk for vaccine related injury. Mumps virus, and certain strains of mumps vaccine (Urabe Am9, Leningrad 3), are among the most neurotropic of the early childhood viruses; New MMR combinations continue to be proposed that include new strains of mumps vaccine virus. Thus, identification of the specific neuronal components that lead to enhanced susceptibility to neurovirulent viruses can lead to sensitive tests for neurovirulence potential during pre-licensing studies. In addition, the development of molecular, in vitro or small animal models of neurovirulence will lead to cost saving and improved predictability of neurovirulence testing.